In order to reduce the number of components and increase the battery life of portable communications devices, many receiver architectures perform direct conversion, which converts a received signal from a radio frequency directly to baseband. Although direct conversion receiver architectures benefit the communication device by decreasing device size and increasing battery life, the performance of the receiver is diminished due to decreased second order intermodulation distortion (IMD2) performance and static and dynamic DC errors.
IMD2 performance of a direct conversion receiver is degraded because the desired output signal of the direct conversion receiver is near DC. IMD2 components are also near DC and are not filtered. Thus, the desired output signal and the IMD2 components are both co-located in frequency and become inseparable.
Direct conversion receivers include local oscillators that generate a frequency equal to or harmonically related to the frequency of the received signal. Because the frequency of the local oscillator is equal to or harmonically related to the frequency of the received signal, there is coupling between the blocker signals and the output of the local oscillator. Thus, during downconversion, the blocker signals are mixed with themselves, thereby producing the square of the blocker signals at DC.
In addition to intermodulation distortion, static and dynamic DC errors degrade the performance of direct conversion receivers. One static DC error is a residual DC offset term due to leakage of the local oscillator into the received signal. Thus, the output of the local oscillator is mixed with itself during downconversion, thereby producing the DC offset term. To correct this static DC error, direct conversion receivers may perform DC correction during baseband processing. However, the DC correction does not correct dynamic DC errors occurring due to thermal drift of the receiver or due to interferers that appear after an initial DC correction time associated with DC correction.
Therefore, there remains a need for a receiver architecture that essentially eliminates all static and dynamic DC errors and improves IMD2 performance.